A herd of elephants congregates at a water hole in Zakouma National Park. The animals' days are generally spent foraging for food in the surrounding savanna. From May to October—the rainy season in Chad—elephants cross the park's borders in search of better forage.

"During the wet season more elephants may be outside of the park boundaries than inside," Fay told National Geographic News. "The corridors they use to leave have been known for a long time, but no one had surveyed outside the park in the wet season."

It did not take Fay long to uncover evidence of large-scale killings on the fringes of Zakouma. His team was in the air less than two hours before they began spotting dead elephants.